72 MARKETABLE BRITISH MARINE FISHES 



sea-water and there acts upon the eggs shed at the same time by 

 the female, and so the eggs are fertihsed. 



The milt both in bony fishes and in skates and dog-fishes 

 appears to be a thick white liquid. It really consists of innumer- 

 able minute solid particles suspended in a transparent liquid. 

 These particles can only be distinguished with the help of the 

 microscope. They are called spermatozoa, or, for the sake of 

 brevity, sperms. The most remarkable fact about them is that 

 when alive and fully developed or " ripe " they are in constant and 

 active motion. Each consists of a thicker, denser portion, the 

 head, to which is attached a very thin long lash or tail, and the 

 motion is due to the incessant vibration of the tail from side to 

 side, which drives the sperm rapidly through the liquid containing 

 it, just as a tadpole is propelled through the water by the move- 

 ment of its tail. The extraordinary activity of the multitude of 

 sperms in a drop of living milt examined under the microscope is 

 an interesting spectacle, and forcibly reminds the observer of a 

 number of animalculae in a drop of water. In fact, when first 

 discovered the sperms were called the animalculae of the semen, 

 and the part they played in the fertilisation of the egg was not 

 understood. 



The head of the sperm differs in shape in different orders of 

 fishes. In the skates and rays it is a long rod having a spiral 

 form, in bony fishes generally it is nearly globular, but in the 

 herring its length is somewhat greater than its breadth. Fig. 33 

 represents the shape of the sperms of certain fishes as seen 

 when highly magnified. The side of each square in the figure 

 represents yitoo inch, so that the extremely minute size of the 

 objects is plainly evident ; the head of the sperm of the thorn- 

 back ray is a little more than yTftro inch in length, that of the 

 herring about s^oVo inch, of the pike a little less, while in the 

 plaice and sole the diameter of the head does not exceed 

 T'o.'o'dT)" ^ncn. 



Fertilisation consists in the entrance of one of these sperms 

 into the egg, where its head unites with the germ, and the latter 

 is then capable of development. It would be utterly out of the 

 question to consider the number of these sperms in the milt of 

 the male fish : it would be as easy to give the number of hairs 

 in a good head of hair, or the number of grains in a ton of sand. 

 But it is a curious fact that only one sperm is needed to 



